Decoupling of magnetically interacting crystallites of goethite

The magnetic properties of synthetic goethite, precipitated from a SiO₂-containing Fe(III) solution, have been studied. The Mössbauer spectra show the coexistence of a quadrupole doublet and a magnetically split component in a large temperature range. The area ratio of these two components depends strongly on temperature, indicating a normal superparamagnetic behaviour of isolated crystallites. This is in contrast to the behaviour normally exhibited by pure samples of microcrystalline goethite, in which strong magnetic interactions among the crystallites lead to a superferromagnetic behaviour. The results show that the magnetic interactions among the crystallites are drastically diminished when amorphous SiO₂ separates the goethite crystallites.